1. Field of the Invention
The present invention relates to an object based video information coding apparatus and method for an MPEG-4 system, and more particularly to an improved object based video information coding apparatus and method capable of compressing image data without reducing image quality by processing motion-incompensable objects into hierarchical image data blocks and transforming the data blocks using a Discrete Cosine Transform.
2. Description of the Conventional Art
Generally, MPEG-4 is used for compressing and reproducing moving images using a spatial correlation of the images. It uses a Discrete Cosine Transform (hereinafter "DCT") to eliminate redundancies in the image, and performs motion compensation.
Referring to FIGS. 1A and 1B, in MPEG-4, images are classified into three types: I-frame images, P-frame images, and B-frame images. In I-frame type images, the images are coded using a discrete cosine transform circuit 106 with no prediction of motion. In P-frame type images, motion is predicted only in the forward direction. In B-frame type images, motion is predicted in both forward and backward directions.
An image at a first input terminal 101 is reconstructed by a first frame reconstruction circuit 102. The motion of the image outputted from first frame reconstruction circuit 102 is predicted by a motion prediction circuit 122. A first motion compensation circuit 121 performs motion compensation using a motion vector 123. An inter/intra analyzing circuit 124 determines whether a macro block being processed is in an Inter Mode, which is a motion compensation mode, or an Intra Mode, which performs a discrete cosine transform without a motion compensation, and controls a first switching circuit 125 accordingly.
The image outputted from first frame reconstruction circuit 102 is outputted to a subtractor 103, which produces a differential image. A frame/field analyzing circuit 104 determines a discrete cosine transform type for the image. A frame/field formatting circuit 105 converts the image data based upon a cosine type determined by frame/field analyzing circuit 104.
The output of the frame/field formatting circuit 105 is converted to a frequency component signal by a discrete cosine transform circuit 106. The discrete cosine-converted frequency component signal is quantized by a quantization circuit 107 and inputted to a zig-zag scan conversion circuit 108. The quantized coefficient value outputted from quantization circuit 107 is scanned in a zig-zag scan by zig-zag scan conversion circuit 108. The output thereof is detected by run-and-fixed length decoder 109 by the number of zeros and levels.
Quantization circuit 107 receives a control signal in accordance with the state of a first buffer circuit 110. If a large number of bit arrays are stored in first buffer circuit 110, the quantization control circuit reduces the number of bits by increasing a quantization scale value. If a small number of bit arrays are stored in first buffer circuit 110, the quantization scale value is decreased and the number of bits in first buffer circuit 110 thereby increases.
The run and level data obtained by zig-zag scan conversion circuit 108 is converted into a bit array corresponding to an MPEG-4 main profile along with additional information such as motion vector 123, and is then outputted to an output terminal 126 at a predetermined bit rate.
In addition to the foregoing, the output of zig-zag scan conversion circuit 108 is decoded through a first inverse zig-zag scan conversion circuit 112, a first inverse quantization circuit 113, a first inverse discrete cosine transform circuit 114, a first inverse frame/field formatting circuit 115, and the decoded output signal is added to a motion-compensated image by a first adder 116 and is thereby recovered.
The recovered images are stored in a first backward reference image memory 120 by a second switching circuit 117 for use in motion prediction and motion compensation, depending upon the coding type of an image received through a second input terminal 118. The stored images are also moved to a first forward prediction reference image memory 119 for when I-frame image or P-frame images are coded.
FIG. 2 shows a conventional MPEG-4 video decoder for recovering an MPEG-4 bit stream for display on a monitor. A bit stream outputted from a third input terminal 201 is stored in a second buffer circuit 202. A run-and-fixed length decoder 203 decodes information about the image, such as discrete cosine transform coefficients, motion vectors, intra/inter mode, and coding type.
The decoded discrete cosine transform coefficient information is recovered by a second inverse zig-zag scan conversion circuit 204. For I-frame images (intra mode), the information is recovered into an image through a second quantization circuit 205 and a second inverse frame/field formatting circuit 207. For P-frame and B-frame images (inter mode) the information is recovered into a differential image. A second inverse discrete cosine conversion circuit 206 is used as well.
The recovered differential image is added to a motion compensation image by an adder 208 and is recovered into an image by a second frame reconstruction circuit 209. The image is then outputted to an output terminal 218 for display.
The recovered image is stored in a second backward prediction reference image memory 215 through a fourth switching circuit 216 in case the coding type of an image input through a sixth input terminal 217 is a B-type image, and is moved from second backward prediction reference image memory 215 to a second forward prediction reference image memory 214 for when an I-frame image or P-frame image is decoded.
The second motion-compensation circuit 212 performs motion-compensation using a motion vector outputted from a fifth input terminal 213, which is obtained by run-and-fixed length decoder 203. The motion image compensated by the second motion compensating circuit 212 is transferred to a second adder 208 through a third switching circuit 210 for processing when an inter mode is detected by an intra/inter mode decision 211 as determined by run-and-fixed length decoder 203.
Conventional MPEG-4 video coders and decoders are disadvantageous in that the images produced by such systems are of low quality because the compressed image data transferred in a coded form is simply recovered to an original image data.